1. Field of the Invention
The present invention relates to a fuel cell electrode, and a membrane-electrode assembly and a fuel cell system including the electrode. More particularly, the present invention relates to a fuel cell electrode for improving a fuel cell life-span by preventing catalyst poisoning, and a membrane-electrode assembly and a fuel cell system including the electrode.
2. Description of the Related Art
A fuel cell is an electrical power generation system that converts chemical energy into electrical energy based on an electrochemical reaction between hydrogen and oxygen in a hydrocarbon-based material such as methanol, ethanol, natural gas, and the like.
Depending on the kind of electrolyte, fuel cells are divided into Phosphoric Acid Fuel Cells (PAFCs), Molten Carbonate Fuel Cells (MCFCs), Solid Oxide Fuel Cells (SOFCs), Proton Exchange Membrane Fuel Cells (PEMFCs), and Alkaline Fuel Cells (AFCs). The fuel cells fundamentally operate on the same principle, but the kinds of fuel, operating temperature, catalyst, and electrolyte are different.
Among the fuel cells, the Polymer Electrolyte Membrane Fuel Cell (PEMFC), which has recently been developed, has excellent output characteristics and fast starting and response characteristics, as well as a low operating temperature, compared to other types of fuel cells. It also has an advantage in that it has wide applications including a distributed power source for houses and public buildings, and a small power source for electronic devices as well as for a transportable power source for a car.
The PEMFC requires a fuel cell body called a stack, which will be referred to as a stack hereinafter for the sake of convenience, and a fuel tank and a fuel pump for supplying fuel from the fuel tank to the stack. It further requires a reformer for generating hydrogen by reforming the fuel while supplying the fuel stored in the fuel tank to the stack and supplying the hydrogen to the stack. The PEMFC generates electrical energy by supplying the fuel stored in the fuel tank to the reformer owing to the pumping power of the fuel pump, generating hydrogen gas through the fuel reformation in the reformer, and causing the hydrogen gas to be oxidized and oxygen to be reduced electrochemically in the stack.
The fuel cell can adopt a Direct Oxidation Fuel Cell (DOFC) scheme to directly supply liquid-phase methanol fuel to the stack. The fuel cell of the DOFC scheme does not require the reformer, which is different from the PEMFC.
In the above fuel cell system, the stack that generates electricity includes several to scores of unit cells stacked in multi-layers, and each unit cell is formed of a Membrane-Electrode Assembly (MEA) and a separator (also collectively referred to as a bipolar plate). The MEA has an anode (fuel electrode or oxidation electrode) and a cathode (air electrode or reduction electrode) attached together with an electrolyte membrane between them. The separator performs a role of a path for supplying hydrogen gas and oxygen, which are required for the reaction of the fuel cell, and a role of a conductor connecting the anode and cathode of the membrane-electrode assembly serially. Through the separator, hydrogen gas is supplied to the anode, whereas oxygen is supplied to the cathode. During the process, the hydrogen gas goes through an electrochemical oxidation reaction at the anode, and the oxygen goes through an electrochemical reduction reaction at the cathode. Due to the transfer of electrons during the reactions, electricity is obtained along with heat and water.
During the fuel cell operation, carbon monoxide (CO) is generated as a by-product and the life-span of the fuel cell is deteriorated due to catalyst poisoning by the carbon monoxide.